With the advent and extreme popularity of smart mobile devices (e.g., the iPhone, BlackBerry, and other Smart Phones), data usage has increased to a point where network congestion caused by bandwidth-hungry devices has led to a looming spectrum crisis that is the biggest threat to the future of mobile telecommunications in America. Evidence of this spectrum crisis is evident, notably at the presidential inauguration in January of 2009, where hundreds of thousands of people gathered to witness the historical event. It is well documented that the cellular network became so congested that no calls were possible. Similarly, cellular network congestion has become a common global occurrence at virtually every major spectator event.
Presently, most wireless networks are optimized to deliver the greatest range and coverage, as this reduces the cost of equipment required for a given area. To achieve this goal, higher radio power, better receive sensitivity, and high gain antennas are all used at the base stations or Access Points (APs). However, in very high-density applications, such as special events, temporary events, emergency events, stadiums, etc., user densities are extremely high; sometimes on the order of users-per-square-meter. In this case, the number of APs should be greatly increased in order to achieve the desired capacity by re-using frequencies throughout the venue. Unfortunately, this increase in density also greatly reduces the probability of interference between APs in the network, thereby nullifying the potential for re-use. In addition, user portable devices (typically erroneously) connecting to APs from long distances, and hence low signal levels, and hence low data rates, should be limited.
Cell-splitting is a common technique used throughout the cellular industry to increase network capacity where available spectrum is scarce. The cellular network derived its name from the deployment techniques used to provide coverage areas. High powered radio frequency transceivers, called macro base transceiver systems (BTS's), were deployed on roof tops and tall towers and by using high gain directional antennas created a contiguous arrangement of “cells” most typically depicted as an hexagonal honeycomb lattice structure. These BTS elements were all interconnected with T1 or T3 wired circuits to central network elements leading to the “cellular network”.
As capacity grew, BTS systems added additional radio frequency channels, while improving the efficiency of the radio signal. First generation BTS employed Analog Mobile Phone System (AMPS) that supported a single cellular user per carrier. AMPS was superseded by Time Division Multiple Access (TDMA) supporting three cellular users per carrier. Cellular technology has steadily progressed with twenty years of improvements and new protocols—GSM, CDMA, 2G, 3G, 4G, new modulation formats, and smart antenna/Multiple In Multiple Out (MIMO) systems—such that the utilization of radio frequency spectrum for the cellular network is now so highly optimized that significant capacity gains are no longer possible using the existing network infrastructure cell locations.
Consequently, cell-splitting techniques have been introduced to further capacity. In addition, Distributed Antenna Systems (DAS) were introduced to provide better coverage in office buildings. More recently, there has been an introduction of limited coverage micro-cells and pico-cells for high capacity applications.
It is evident that the evolution of the cellular/wireless industry has shifted from larger macro-cells to smaller pico-cells with greater improvements in spectral and spatial efficiency of licensed band cellular networks.
Unlicensed band networks have evolved in a somewhat divergent path, due to the impact of multiple wireless services sharing the same bands and the requirement for improved coverage. Unlicensed band networks, such as Wi-Fi, were the first to incorporate advanced spectral and spatial techniques to achieve high levels of spectral and spatial efficiency. However, unlicensed band networks have grown, in contrast to the licensed band cellular networks trends, by starting from very small cells limited by low transmitter power, typically 25 mW (14 dBm), to achieve cell sizes sufficient to cover a house. They have improved to the point of meeting the Industrial Scientific Medical (ISM) band regulatory limits 4 W (36 dBm) to provide coverage of large hot spots such as malls or train stations.
Unlicensed band products are also divergent in their network planning/adaptation techniques. Cellular network cell sizes are defined by the transmitted power, modulation formats used, or in the case of CMDA systems, by the spreading codes employed, and have been engineered by network designers and, more recently, by automated software tools used to optimize cell locations and sizes. Unlicensed cells have been autonomous in nature with no central control mechanisms to set the cell sizes. Each unlicensed transceiver has employed techniques to achieve the maximum cell size at the highest transmitter power available. Wi-Fi radios have employed spectral (modulation rate adaptation) and spatial (e.g., maximal ratio combining and spatial time block coding) techniques to achieve the greatest possible cell sizes.
Modulation rate adaptation algorithms are well documented in the Wi-Fi industry to achieve these goals of maximum coverage at the highest throughput. These algorithms are designed for Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocols, where if a transmitted packet is not acknowledged, the transmitter adjusts/reduces the modulation rate and retransmits the packet again at the same maximum allowed power level. Rate adaptation algorithms are provided by the wireless chip manufacturers and form the de-facto operation of all Wi-Fi and wireless devices, so that all devices behave similarly to achieve the maximum coverage at the highest modulation rate.
These algorithms work well in standard wireless networks, but do not work well in very high capacity venues which are interference limited, such as sports stadiums, outdoor concerts, emergency events, temporary events such as carnivals, theme parks, and some very high density urban environments, where the user densities may be measured on the order of users per square meter.